全文获取类型
收费全文 | 6019篇 |
免费 | 210篇 |
国内免费 | 10篇 |
专业分类
化学 | 4350篇 |
晶体学 | 50篇 |
力学 | 126篇 |
数学 | 544篇 |
物理学 | 1169篇 |
出版年
2023年 | 54篇 |
2022年 | 58篇 |
2021年 | 218篇 |
2020年 | 166篇 |
2019年 | 172篇 |
2018年 | 151篇 |
2017年 | 100篇 |
2016年 | 241篇 |
2015年 | 219篇 |
2014年 | 263篇 |
2013年 | 379篇 |
2012年 | 452篇 |
2011年 | 495篇 |
2010年 | 311篇 |
2009年 | 312篇 |
2008年 | 402篇 |
2007年 | 335篇 |
2006年 | 312篇 |
2005年 | 243篇 |
2004年 | 211篇 |
2003年 | 152篇 |
2002年 | 123篇 |
2001年 | 98篇 |
2000年 | 88篇 |
1999年 | 58篇 |
1998年 | 51篇 |
1997年 | 49篇 |
1996年 | 38篇 |
1995年 | 41篇 |
1994年 | 43篇 |
1993年 | 44篇 |
1992年 | 46篇 |
1991年 | 25篇 |
1990年 | 28篇 |
1989年 | 18篇 |
1988年 | 21篇 |
1987年 | 15篇 |
1986年 | 13篇 |
1985年 | 16篇 |
1984年 | 19篇 |
1983年 | 19篇 |
1982年 | 11篇 |
1981年 | 22篇 |
1980年 | 11篇 |
1979年 | 12篇 |
1978年 | 16篇 |
1977年 | 9篇 |
1976年 | 9篇 |
1975年 | 9篇 |
1973年 | 8篇 |
排序方式: 共有6239条查询结果,搜索用时 545 毫秒
71.
C. Gonalves Joaquim C.G. Esteves da Silva M.F. Alpendurada 《Analytica chimica acta》2006,560(1-2):164-171
An extensive monitoring programme of pesticides was carried out in soil samples from an intensive horticulture area in north of Portugal, putting into practice the needs for increased control of soil quality as far as organic pollution is concerned. The area under investigation was additionally defined as vulnerable to nitrates due to local soil and aquifer characteristics, which might be extended to pesticides contamination. Five sampling sites were selected and soils analysed at three depths in eight sampling campaigns, for the period of 2 years. A stepwise multivariate statistical approach was selected to uncover most relevant patterns inside a complex environmental data matrix. Cluster analysis was applied both to group pesticides and samples, giving a primary and unsupervised overlook of privileged relationships. Clusters of persistent pesticides and selected herbicides were identified, whereas sample classes were also formed and disposed geographically. Thirty eight percent of analysed soils samples fell into one class characterized by low contamination (class 1 in cluster analysis), which is entirely representative of the sampling site no. 1. Afterwards, linear discriminant analysis was applied to identify those pesticides, which had a higher impact in the definition of classes. Finally, factor analysis using a five component model was implemented in order to bring to light the constitution and data variance explained by each of the five main principal components, as well as, their relation to pest management practices. A factor was identified (PC1 – 22% variance) composed of chlorinated pesticides, which was representative of one of the investigated sites indicating its high contamination status. Qualitative main findings and class average concentration values were obtained through this multivariate statistical approach. 相似文献
72.
This paper presents a method for the energy analysis of alpha-particles emitted by transuranic elements via a novel technique
using Nuclear Tracks. The method is based on the relationship between the energy deposited in the detection material and the
diameter of the track, that is formed by chemical etching. The method involves CR-39 polycarbonate as the detector material,
one-step chemical etching after irradiation, and a digital image analysis system for automatic reading of the track diameters.
The experimental study included alpha-particles in the energy range 5.1 MeV to 5.8 MeV emitted by239Pu,241Am and244Cm. The quantitative results provide a clear signature to identify each one of the emitters based on a characteristic track
diameter. 相似文献
73.
A simple route for the enantioselective synthesis of key intermediates (11 and 12) for the total synthesis of forskolin has been developed starting from acid 6 and (S)-alcohol 5. The latter is prepared by enantioselective catalytic CBS reduction of dienone 3, and is converted by an intramolecular Diels-Alder reaction to tricyclic lactone 9. 相似文献
74.
A more consistent, straightforward, and economical protocol for generation of stannylene species and their reaction with BnBr leading to products of O-monobenzylation of diols has been set. It has shown to be specially indicated for substrates bearing vicinal trans 1,2-diol moieties on cyclohexane backbones, which are more resistant to these transformations. Such protocol has been successfully applied to myo-inositol derivatives and acyclic diols. 相似文献
75.
Summary The synthesis, characteristics and analytical applications of pyridoin phenylhydrazone are described. It reacts with copper(I) to produce an orange 11 complex (
max=450 nm,=2.1×104l· mole–1·cm–1) in aqueous ethanolic solution; it behaves as a cuproin type reagent. The orange complex can be extracted into amyl alcohol (
max=440 nm,=2.0×104l·mole–1·cm–1), and used for the spectrophotometric determination of trace amounts of copper. Interferences have been investigated.
Zusammenfassung Synthese, Eigenschaften und analytische Anwendung von Pyridoin-phenylhydrazon wurden beschrieben. Es reagiert mit Cu(I) unter Bildung eines orange gefärbten 11-Komplexes in wäßrig-alkoholischer Lösung ( max=450 nm,=2,1×104l·mol–1·cm–1); das Reagens verhält sich ähnlich wie Cuproin. Der erwähnte Komplex läßt sich mit Amylalkohol extra-hieren ( max=440 nm,=2,0×104l·mol–1·cm–1) und zur spektrophoto-metrischen Bestimmung von Kupferspuren benützen. Auftretende Störungen wurden angegeben.相似文献
76.
J. L. Q. de Britto A. M. S. Braghirolli A. G. da Silva 《Journal of Radioanalytical and Nuclear Chemistry》1985,96(2):181-186
A new and simplified method was developed for the separation of thallium 201 which will allow the shipment of a “one shot generator” to distant places and easy elution at the destination. Thallium is produced as usually, through the reaction $${}^{nat}Tl/p,xn/^{201} pb\xrightarrow{{\beta ^ + }}{}^{201}Tl$$ with 24-MeV protons from IEN'S CV-28 cyclotron. The separation is based on the properties of a chelating carboxylic acid cation exchange resin column which at pH 4.5 retains lead while thallim is easily eluted. This column can also be used as a regular generator to produced greater quantities of201Tl but at the expense of more elutions. 相似文献
77.
S.A.Vieira Filho L.P. Duarte G.D.F. Silva O.W. Howarth I.S. Lula 《Helvetica chimica acta》2003,86(10):3445-3449
3β‐(Stearyloxy)olean‐12‐ene was isolated from a hexane extract of Austroplenckia populnea Reiss (Celastraceae) leaves. The structure was solved by means of quantitative 13C‐NMR, HMBC, HMQC, COSY, NOESY, and NOE difference spectra. The mass spectrum showed an [M+1]+ ion peak at m/z 693, and the molecular formula C48H84O2 was confirmed by combustion analysis. 相似文献
78.
J. M. Hernndez Ana M. Herrera J. García‐Serrano J. F. Rivas‐Silva 《International journal of quantum chemistry》2002,88(3):342-346
Conformations of p‐methacryloylaminophenylarsonic acid (p‐MAPHA) are determined through molecular mechanics and DFT/B3LYP calculations. Solvation effects are studied within the self‐consistent isodensity continuum model (SCI‐PCM). The stationary points were found to correspond to minima as verified by the analysis of vibrational frequencies in the molecule. The molecular optical absorption was obtained by using different solvent environments. The present results are in good agreement with the available experimental data. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002 相似文献
79.
Anne M. Garrido Pedrosa M. J. B. Souza A. O. S. Silva Dulce M. A. Melo A. S. Araujo 《Journal of Thermal Analysis and Calorimetry》2006,84(2):503-509
The study of the incorporation
of rare earth elements as additives in Y zeolites is a very interesting field
of research, mainly by its potential application as additives in catalytic
cracking process. In this work was studied the thermal and structural properties
of cerium, holmium and samarium supported on HZSM-12 zeolite. The obtained
materials were characterized by X-ray diffraction (XRD), infrared spectroscopy
(FTIR), nitrogen adsorption, thermogravimetry (TG/DTG), differential scanning
calorimetry (DSC) and differential thermal analysis (DTA). TG/DSC/DTA analyses
showed that the dehydration temperatures of RE/HZSM-12 zeolites (RE=Ce, Ho, Sm) increase in relation to pure HZSM-12.
The acid properties were investigated by pyridine thermo desorption via TG.
The results showed two events of mass loss attributed to elimination of pyridine
adsorbed on the weak+medium acid sites and on the strong acid sites. 相似文献
80.
Maria Fernanda Silva Estela Soledad Cerutti Luis D. Martinez 《Mikrochimica acta》2006,155(3-4):349-364
The purpose of this article is to review and evaluate cloud point extraction of metals and its coupling to different contemporary
instrumental methods of analysis. This review covers a selection of the literature published on this topic over the period
mainly between 1997 and September 2005 (consisting of 50 publications). The current state of the art for CPE concerning metals
and metal chelates is presented with special emphasis on the hyphenation of this interesting extraction/preconcentration approach
mediated by surfactants to spectrophotometry, atomic spectroscopy and separation techniques. We present contemporary CPE developments
concerning metal speciation and determination and their application to different environmental, clinical, geological and food
samples. Strategies for method development as well as future perspectives are also covered. 相似文献